taildrop: lazily perform full deletion scan after first taildrop use (#10137)

Simply reading the taildrop directory can pop up security dialogs
on platforms like macOS. Avoid this by only performing garbage collection
of partial and deleted files after the first received taildrop file,
which would have prompted the security dialog window.

Updates tailscale/corp#14772

Signed-off-by: Joe Tsai <joetsai@digital-static.net>

taildrop/delete_test.go

@@ -11,6 +11,8 @@ import (
 	"time"
 
 	"github.com/google/go-cmp/cmp"
+	"tailscale.com/ipn"
+	"tailscale.com/ipn/store/mem"
 	"tailscale.com/tstest"
 	"tailscale.com/tstime"
 	"tailscale.com/util/must"
@@ -72,6 +74,8 @@ func TestDeleter(t *testing.T) {
 	m.opts.Logf = t.Logf
 	m.opts.Clock = tstime.DefaultClock{Clock: clock}
 	m.opts.Dir = dir
+	m.opts.State = must.Get(mem.New(nil, ""))
+	must.Do(m.opts.State.WriteState(ipn.TaildropReceivedKey, []byte{1}))
 	fd.Init(&m, eventHook)
 	defer fd.Shutdown()
 	insert := func(name string) {
@@ -85,8 +89,8 @@ func TestDeleter(t *testing.T) {
 		fd.Remove(name)
 	}
 
-	checkEvents("start init")
-	checkEvents("end init", "start waitAndDelete")
+	checkEvents("start full-scan")
+	checkEvents("end full-scan", "start waitAndDelete")
 	checkDirectory("foo.partial", "bar.partial", "buzz.deleted")
 
 	advance(deleteDelay / 2)
@@ -134,3 +138,15 @@ func TestDeleter(t *testing.T) {
 	remove("wuzz.partial")
 	checkEvents("end waitAndDelete")
 }
+
+// Test that the asynchronous full scan of the taildrop directory does not occur
+// on a cold start if taildrop has never received any files.
+func TestDeleterInitWithoutTaildrop(t *testing.T) {
+	var m Manager
+	var fd fileDeleter
+	m.opts.Logf = t.Logf
+	m.opts.Dir = t.TempDir()
+	m.opts.State = must.Get(mem.New(nil, ""))
+	fd.Init(&m, func(event string) { t.Errorf("unexpected event: %v", event) })
+	fd.Shutdown()
+}